python: pip executable be at HOMEBREW_PREFIX/bin

Removed the `--install-script=#{bin}"` arg to the python setup.py call
and also moved that call _after_ we wrote the `distutils.cfg`. The reason
for this is that `pip` should end up in `HOMEBREW_PREFIX/bin/`. This is
no longer the case since the install of pip was moved to `post install`,
because there is no more symlinking from cellar to prefix.